In early March 2020, at a campaign stop in Michigan, Biden commented, "I view myself as a bridge, not as anything else," and then referenced a new generation of leaders that was waiting in the wings. That's not an explicit declaration of a one-term presidency, but it's pretty darned close. I think most people assumed he'd be one (term) and done.
I just checked out a thread from two years ago where I predicted that Biden wouldn't run for reelection, that there would be a crowded and competitive field of Dem candidates, and that Harris would not emerge as the nominee. That wasn't a particularly bold prediction, and there was plenty of agreement.
In accordance with his March 2020 vision, Biden needs to end his campaign for reelection and let a new generation take over. It's pretty late in the game, I think they have to nominate someone other than Harris (because I don't think she can beat Trump) and that may not pretty, I'm not sure how they get to a consensus candidate, but they've got to figure out a way. As someone said last night, this is politics and they can make up or change the rules as they go along.
I believe Whitmer, governor of a key state, would beat Trump. I think Kelly, Senator from a key state, would beat Trump. There are other candidates as well who would destroy him on a debate stage and beat him in November. Gotta get moving, Dems.